1,720,986 research outputs found
Sequenza di impulsi per acquisire dati di risonanza magnetica nucleare in funzione del rilassamento longitudinale
No full textL' invenzione introduce una nuova categoria di sequenze di impulsi a radiofrequenza, chiamata PERFIDI (Parametrically Enabled Relaxation Filters with Double and multiple Inversion), particolarmente utile per lo studio a Risonanza Magnetica Nucleare (NMR) di sistemi che presentino una distribuzione di tempi di rilassamento longitudinale. La categoria di impulsi a radiofrequenza agisce come un filtro, estraendo il segnale che viene generato da componenti del campione, il cui tempo di rilassamento longitudinale giace in un determinato intervallo di valori.
Le varie componenti del segnale vengono filtrate gia' nelle fasi di acquisizione, esaltando la capacita' di discriminazione. Essendo definita parametricamente (con i ritardi tra gli impulsi che agiscono come parametri aggiustabili) la forma funzionale del filtro puo' essere in larga misura controllata dall' operatore dello strumento NMR/MRI
PERFIDI: Parametrically Enabled Relaxation Filters with Double and Multiple Inversion
No full textWe present a novel approach to the nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) analysis of complex
samples with nontrivial distribution of longitudinal relaxation rate R1. Parametrically enabled relaxation filters with double and multiple
inversion (PERFIDI) aim to separate signals arising from components with different R1 values prior to actual data acquisition. Given any standard NMR/MRI pulse sequence, which, by itself, is insensitive to differences in R1 values, it can be combined with a PERFIDI preamble, which functions as a preliminary R1 filter and confers on the original technique sensitivity to the dimension R1. This article states the principles of the approach, including the way to account for instrumental imperfections, and shows how PERFIDI with specific filter profile functions can be built. Using terms borrowed from electronics, these filters are classified as low-pass, high-pass and band-pass types. Also included are an experimental verification example and a discussion of potential applications of PERFIDI in various NMR areas
Optimization of NMR sequences through simulation of real samples and instrumental artifacts: application to the LAPSR sequence
No full textPERFIDI: una classe di sequenze NMR operanti come “filtro” a priori per sistemi multicomponenti
No full textNew Universal NMR Sequences: PERDIDI and LAPSR
No full textThis work presents two new families of sequences developed within two different projects at the University of Bologna. The two families are related by a common feature, which is the use of 180o inversion pulses as a preamble to a classical NMR technique. The exclusive use of inversion pulses confers them a remarkable insensitivity to offset-related artifacts, a property particularly appreciated in NMR relaxometry of large, complex samples and in MR imaging. The final aims of the two families, however, are somewhat different:
PERFIDI (Parametrically Enabled Relaxation Filters with Double and multiple Inversion) is a family of relaxation filters applicable as a preamble to almost any NMR pulse sequence. As such, it will find a wide range of applications in all branches of NMR, including spectroscopy, relaxometry and imaging. It has been designed keeping in mind complex, chemically and/or physically heterogeneous systems such as untreated body fluids, biological tissues and whole organs, porous media, etc. The patented sequences (patent BO2005A000445, University of Bologna), comprised of n inversion pulses (n = 2,3,4,...) are crafted to provide (borrowing electronics terminology) high-pass, low-pass and band-pass T1-filters of various shapes and cut-off sharpness.
LAPSR (Logarithmically distributed A-Periodic Saturation Recovery) is one of a family of SMS (Sample Magnetization Suppression) sequences whose aim is to suppress as fast as possible the nuclear magnetization of all components of a sample. A particular attention is paid to samples with wide distributions of relaxation times (e.g., from 0.3 ms to 3 s), offsets, and nutation angles (B1 inhomogeneity). The development of SMS sequences started from the observation that:
(i) Classical methods of NMR relaxometry such as inversion recovery (IR) are rather slow because they require reaching the equilibrium magnetization before every scan. In addition, they fail in situations where sample complexity combines with severe and unavoidable instrumental imperfections (ex-situ NMR, large samples and coils, severe B1 inhomogeneity, insufficient transmitter power, etc).
(ii) The alternative is to use the saturation recovery sequence (SR) or the APSR sequence (one composed of 90o pulses with linearly decreasing delays), possibly in combination with gradient pulses. The goal is to achieve the zero-magnetization starting state and do so in a fast and reproducible way. The results are in fact often better than using IR but, nevertheless, still far from being free of artifacts.
These observation prompted us to start an extensive series of theoretical simulations trying to answer the question of how fast and how well can one suppress the magnetization of complex samples using standard pulse sequences. The theoretical results, by themselves very interesting, were then compared with experiments. It turns out that the best sequences in this category are composed of a large number (15-20) of inversion pulses (nominally 180o) with logarithmically decreasing delays. Relaxation curves can be obtained about 3 times faster than using IR and they remain meaningful even in experimental conditions which, from the NMR point of view, appear quite unsatisfactory
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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